阅读短文，选择正确的答案。

    Mrs. Jones was a teacher. Her house was not far from her school, and she often walked there in the morning. All the students in the school were very young.

    One cold and windy morning in October, Mrs.Jones walked to school. The cold wind went into her eyes and big tears（泪）began to run out of them. She reached (到达) the school, opened the door and went into the classroom.

    It was nice and warm there and Mrs. Jones was happy. But then a small boy looked at her for a while, put his arms a round her and said (说):"Don't cry, Mrs. Jones. School isn't very bad. "

阅读材料，完成有关问题。

青海湖流域位于青藏高原东北部内流区，流域内环境独特。下图示意青海湖流域。

$\begin{array}{c}\frac{a_1+a_2}{2}\ge \sqrt{a_1{a}_2}\\ \frac{a_1+a_2+a_3}{3}\ge \sqrt[3]{a_1{a}_2{a}_3}\\ \frac{a_1+a_2+a_3+a_4}{4}\ge \sqrt[4]{a_1{a}_2{a}_3{a}_4}\\ \cdots \cdots \cdots \end{array}$  

照此规律，当 $n\in N^{+}\left(n\ge 2\right)$  时， $\frac{a_1+a_2+a_3+\cdots +a_n}{n}\ge$ ____．